The disclosed invention relates to systems for transmitting propagating wave energy from an array of transducers and determining the direction to the source of the wave energy or to a target reflecting such energy from the frequency of the wave motion. Known systems use frequencies ranging from those in the sound spectrum, for example sonar, to those in the electromagnetic spectrum, such as radar.
Systems containing arrays of transducers such as the linear or planar arrays utilized in sonar, sound detection and ranging of targets under water, have long been used. Conventional sonar systems are hampered by problems such as ambiguity in determining the direction of a target and reverberation. Normally, if a target echo is detected by a line receiving array, the direction to the target has two possible solutions, generally called left-right ambiguity. Detection of the echo on two widely-separated, horizontal line receiving arrays must be made simultaneously to resolve the ambiguity. Even this method of localizing is not always possible since signals are not always received simultaneously.
When conventional, long duration, large time-bandwidth product sonar signals are radiated, there is a very large area of ocean contributing to reverberation in which a target echo must be detected at any instant of time. Reverberation is a strong echo from the sea surface, sea bottom, schools of fish or plankton, which arrives at the receiver at the same time as the target echo.
A conventional beam-formed source array is able to discriminate against reverberation, but it can search in only one direction at a time. Some conventional time domain beam-forming arrays essentially create a narrow, focused beam from an array of transmitting elements and steer the beam in a given direction by changing the frequency of the transmitted signal. Other systems sequentially transmit multiple narrow beams in different directions. In conventional beam-forming systems, signals at successive radiating elements are delayed by the time required to bring them into time synchronism as they are radiated in a particular direction. Such systems can search for a target over a wide range of angles, but although such systems may be able to overcome localization ambiguities, they have inherently slow search rates.